In general, a polarization state of an optical signal propagating through a multi-repeater transmission system is an elliptically polarized wave which has a major axis and a minor axis perpendicular to the major axis. When an optical amplification repeater is operated under a gain saturation state, and the powers differ between directions along the major and minor axes of the polarized-light of signal, the gain saturation state varies in the direction of polarization of light due to the power difference. As a result, the gain becomes unequal, and thus the amplification signal power and noise power vary according to the polarization state of the optical signal input into the optical amplification repeater.
Accordingly, a recent multi-repeater transmission system uses an optical component with an extremely small polarized-light dependent optical loss as an optical amplification repeater, while making polarization scattering of a transmission optical fiber small, thereby reducing polarized-light dependency of a transmission line. On the other hand, optical amplification repeaters are used in a gain saturation state while inserting them in an optical fiber transmission line at regular intervals, thereby automatically compensating for transmission loss of the light signal caused by the optical fiber.
Since a state of polarized-light in the optical fiber transmission line fluctuates at points of time according to changes of the external environment (e.g., temperature), the polarization state of the input signal into the optical amplification repeater also changes regularly. Therefore, if the optical amplification repeater has the above-described property, although the polarized-light dependency of the transmission line is reduced, the amplified light signal power and the noise power may fluctuate according to the input polarization state. Fluctuations in light signal power to accumulated noise power ratio (S/N ratio) after transmission changes the receiving characteristic of the light signal. This phenomenon becomes significant as the number of the amplification repeaters incorporated in the multi-repeater transmission system is increased.
In order to reduce such an effect caused by polarized-light dependency of the input signal into the optic amplification repeater, a technique is known in which a polarization scrambler, for example, as shown in FIG. 12(1) is used. By inserting this polarization scrambler on the output side of the light source, the polarization of the signal can be scrambled, by which each polarized-light component of the optical amplification repeater can uniformly be activated to make the amplification gain equal with respect to the polarized-light axis. Specifically, in FIG. 12(1), signal light from a light source 101 with any polarization is guided by an optical fiber 102, and branched into two lines by a 3dB fiber coupler 103. One of the signals is subjected to frequency modulation by an acousto-optic modulator 104. Then, both of the signals are made into linearly-polarized waves by polarization controllers 105a and 105b including a ½λ plate and a ¼λ plate, respectively, precisely adjusted such that both optical powers and phases are equal and same (not shown), and combined by a polarized-light combiner 106. As a result, from polarized-light combiner 106, an output signal is obtained which has been polarization modulated through frequency modulation by the acousto-optic modulator 104 that performs modulation from linear polarization to circular polarization and again to linear polarization.
In a conventional multi-repeater transmission system, a polarization scrambler 2 is inserted immediately before a transmission line fiber 4-1 as shown in FIG. 12(2). In this multi-repeater transmission system, a transmitter 1 and a receiver 2 are connected via the transmission line fibers 4-1, 4-2 . . . , and a plurality of optical amplification repeaters 5-1, 5-2 . . . are inserted between the transmission line fibers 4-1, 4-2 . . .
However, conventional technique has the following problems.
According to the conventional technique, a failure polarization scrambler has to be exchanged for a new one, which is troublesome and renders the multi-repeater transmission system unavailable until the exchange is completed.
In a conventional multi-repeater transmission system, gain polarized-light dependency of the optical amplification repeaters is accumulated due to multistage connection. The only solution to this problem is to reduce gain polarized-light dependency for each optical amplification repeater. However, an extended length of a recent transmission line has increased the number of optical amplification repeaters, requiring higher performance of each optical amplification repeater. As a result, the gain polarized-light dependency per optical amplification repeater has reached the limit.